1. Field of the Invention
The present invention relates to an electrophotographic photoreceptor. More particularly, the present invention relates to an electrophotographic photoreceptor having a high sensitivity, which comprises a photosensitive layer containing an organic photoconductive material.
2. Description of the Prior Art
Heretofore, an inorganic photoconductive material such as selenium, cadmium sulfide or zinc oxide has been widely used in a photosensitive layer of an electrophotographic photoreceptor. However, these inorganic photoconductive materials have disadvantages that selenium and cadmium sulfide must be recovered since they are toxic materials, that selenium is poor in heat resistance since it is crystallized by heat, that cadmium sulfide and zinc oxide are poor in moisture resistance, and that zinc oxide is poor in printing resistance, and accordingly an effort for developing a novel photosensitive material has been continuously made. Recently, a research for using an organic photoconductive material in a photosensitive layer of an electrophotographic photoreceptor has been progressed, some of which have been practically utilized. As compared with an inorganic photoconductive material, an organic photoconductive material has advantages that it is light, that a film can be easily formed, that it is easy to prepare a photosensitive material, and that it is possible to produce a transparent photosensitive material depending on its kind.
Recently, a separate function type photosensitive material in which a function of generating a charge carrier and a function of transporting the carrier are separately carried on respective compounds, has been mainly developed, since it is effective for providing a high sensitivity, and accordingly, this type of organic photosensitive material has been practically utilized.
As a charge carrier transporting medium (hereinafter referred to as "CTM"), there are a case of using a high molecular photoconductive compound such as polyvinylcarbazole and a case of dispersing and developing a low molecular photoconductive compound in a binder polymer.
Particularly, an organic low molecular photoconductive compound can easily provide a photosensitive material having excellent mechanical properties since a polymer excellent in film-formability, flexibility and adhesiveness can be selected as a binder (see, for example, JP-A-63-172161, JP-A-63-174053, JP-A-4-267261 and JP-B-5-15259).
Examples of performances required for an electrophotographic photoreceptor include (1) a chargeability by corona discharge in the dark is high, (2) a decay of a surface potential by corona charge in the dark is small, (3) a decay of a surface potential by light exposure is large, (4) a residual potential after light exposure is small, and (5) when repeatedly used, variation in a surface potential, lowering of a sensitivity and accumulation of a residual potential are small, and consequently a satisfactory durability can be provided.
Particularly, when a residual potential is large, a charge remains also on exposed parts, and when development by a toner is made, the toner is developed also on non-image parts, which causes a fogged image. Further, in a case of reverse development used often in a printer, an image density or a contrast is lowered, and in an extreme case, a toner is not deposited on an image part, which causes a void image. In these cases, reproducibility of an image is extremely lowered, and is not practically usable. Recently, in proportion to the spread of a laser printer employing a reverse development system, there is strongly demanded development of CTM having a high sensitivity, a low residual potential, a high mobility and a satisfactory durability, which is suitable for combining with a charge generating material for a long wavelength light such as a phthalocyanine type pigment.